Martin Strauss, via Garmin and Microsoft

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Made by Polar, Garmin, Nike and Timex, global positioning system watches track the distance you have run and your pace, including your average pace and your instantaneous pace. They beep at intervals, like every mile, if you want to train by doing some segments of your course at a faster pace. And when you are finished running, you can download all your data onto your computer.

But after a while, I noticed something disconcerting. My watch might record my run as, say, six miles, but according to Google Maps, the actual distance was more like 6.5 miles.

That kind of discrepancy, of course, plays havoc with your training. The pace calculated by the watch is much too slow, and the run becomes an exercise in frustration.

So I got another watch, from a different maker. It was just as bad, maybe worse. I returned it and got a third one, but that one seemed to be absolutely accurate only once, when I was running along the lakefront in Chicago, under a clear sky with no tall buildings and few trees nearby.

On Sunday, I tried a little experiment with friends who also have GPS watches. I started from my house, and Jen Davis and Martin Strauss started from her house; we met up along the way.

My route was 15.96 miles, according to Google Maps. My watch said it was 15.54. Jen’s watch, an older model, did much better. Her route was 19.1 miles. Her watch said 19.02.

Race organizers know this problem all too well. Douglas Thurston, operations director for the Competitor Group, which organizes Rock ’n’ Roll Marathons, a series of races across the country, braces himself for complaints with every race.

Runners who wore GPS watches start e-mailing him or posting comments on Facebook or Twitter afterward. The course was measured incorrectly, they will say. According to their GPS devices, it was too short.

Mr. Thurston has gotten so used to the complaints that he actually has a generic e-mail reply. No, it says, the course was not wrong. Your GPS device was.

“If someone wants to go to mat on it, I ask them to go to a 400-meter track and run on the inside lane for 12.5 laps. That’s 5,000 meters,” he said in an interview. Then, he tells the runner, check the distance on your GPS device. He guarantees it will not be 5,000 meters.

Martin illustrated this for me recently by running five times around a track at the University of Michigan, where he is a professor of mathematics and electrical engineering and computer science. When he downloaded the GPS data onto his computer, every loop around the track was a little different, and none were oval.

In fact, not one of his paths was even curved — they were short segments of lines connected to resemble an oval. Yet he had run in the same lane.

It seems clear enough that a GPS watch is not very accurate, yet online runners’ forums, like one at the Web site of Runners World, are filled with comments from confused athletes who rely on the devices. One poster, for example, ran a half marathon and wore a GPS watch that said the distance was 12.8 miles instead of 13.1.

“Many people are posting on the race’s Web site that theirs came up just as short,” the runner wrote. “I got a pretty stellar PR” — personal record — “and would hate to have a question mark hanging over it.”

Another wrote, “I did an out-and-back run on a rail trail: 5.25 miles out and 5.02 miles back. According to the GPS, I was running 40 m.p.h. for over two minutes.”

What’s wrong with those GPS devices? The problem, say their makers, is that people expect too much. The watches are very much a work in progress. “We all use pretty much the same technology,” said Corey Cornaccio, director of marketing at Polar. The technology is improving, but some inaccuracy remains. “People don’t understand that,” he said.